Pollutant Information: Nitrogen Oxides

About Nitrogen Oxides

Category: Air pollutants

Studies have shown that oxides of nitrogen (NOx) can cause lung irritation as well as lowering people's resistance to pneumonia and bronchitis and other respiratory infections. In the presence of sunlight, NOx can react to produce a photochemical smog. If hydrocarbons are also present ozone can be produced, which has a similar health effect to NOx. Although higher concentrations of NOx are found in city areas, resulting ozone concentrations tend to be higher in rural areas, where crop yields can be reduced as a result.

Road transport was the most dominant source of nitrogen oxides (NOx) in the UK in 2015, with industrial combustion and power generation also accounting for a large fraction of the emission total. Around a third of the UK NOx emissions in 2015 arose from road transport, most of which came from diesel vehicles. The estimation of these emissions is complex since the nitrogen can originate from either the fuel or atmospheric nitrogen. The emission is dependent on the conditions of combustion, in particular temperature and excess air ratio, which can vary considerably. Thus combustion conditions, load and even state of maintenance are important. Since 1970, overall NOx emissions have decreased by 69%, although this decrease has not been constant (see plot below). Up to 1984, the NOx emission profile was relatively flat with small peaks in 1973 and 1979, which were largely due to the cold winters in those years. From 1984, emissions rose markedly as a result of the growth in road traffic and an increase in the proportion of cars running on diesel, reaching a peak in 1989. Since then, total emissions have declined by 69% mainly as a result of emission reductions observed from power stations and the road transport sector. Emissions from road transport fell due to the introduction of catalytic converters and stricter regulations (i.e. Euro Standards), and power sector emissions have declined due to changes in fuels burnt (more gas and less coal), and the use of control technologies to minimise emissions. The UK is currently on track to meet the Gothenburg Protocol target in 2020, but emissions will need to be reduced by a further 21% from 2015 to meet this target.